In a typical xDSL deployment, a Central Office (CO) side transceiver will communicate with a compatible unit located at the Customer Premise (CP) known as the CP device or equipment (CPE). The service provider can, at times, run diagnostic tests to determine if there have been any changes to, or impairments in, the loop environment since the last diagnostic test was run or if the current line is capable of supporting higher data rates through use of a different xDSL standard, for example, using VDSL versus ADSL. One popular diagnostic test that aims at making such determinations is SELT.
A SELT test consists of transmitting a known test signal and analyzing the return, or echo, signal seen at the transceiver input. The transmit signal will undergo shaping as it traverses the loop. This shaping is representative of the loop characteristics. For example, if a bridge tap is present somewhere on the line under test, a SELT test may be able to identify this through knowledge of the unique signature that a bridge tap presents to the line when probed with a SELT test signal.
Since both ends of the connection are capable of transmitting a SELT signal, either side can run a SELT test. There are advantages to running SELT at one side or the other side of the connection. For determining the loop characteristics of in-home wiring, for example, execution of the SELT test from the CP side is desirable. Conversely, identification of loop impairments in close proximity to the central office is best seen through a CO side SELT test. In either event, since the SELT transmit signal undergoes attenuation that is proportional to the length of loop that it traverses, there are practical limits to which this test can be effectively used to determine the presence of loop wiring impairments.
Accordingly, there remains a need for improved SELT test techniques that address these shortcomings, among others.